Credit Risk and Fair Value of Financial Instruments Level 4 (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Jun. 30, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Bank credit facility due March 2016	$ 0	$ 86,600	$ 86,600
Mortgage notes and other debt, maturities through 2047	4,711		5,698
Total fair value of debt instruments	2,221,612		1,991,339
February 2013 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	0		4,786
April 2015 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	145,676		150,112
April 2016 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	216,621		222,049
June 2017 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	327,450		341,094
October 2018 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	284,560		298,750
May 2019 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	266,875		276,250
November 2020 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	190,500		204,500
November 2021 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	170,250		186,000
January 2022 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	407,469		0
April 2027 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	$ 207,500		$ 215,500